Typically, a color image reproduction system, as shown in FIG. 1, includes a color scanner 20, a digital image processing computer 22, and a color printer 24. The scanner 20 scans a color image to produce a color digital image usually expressed in a red, green, and blue (RGB) color coordinate system. The digital image processing computer 22, which may be a general purpose digital computer or a special image processing computer, transforms the RGB digital image to a form that can be used by the printer 24, such as a cyan, magenta, and yellow (CMY) digital image. Often the printer 24 will also have black printing capability, and the digital image signal sent to the printer will include a black component K (CMYK). The digital image processing computer 22 also performs any desired color balance or tone scale adjustments to the digital image, and performs calibration corrections for the individual properties of the scanner, the printer and the print media used in the printer. All of these adjustments, corrections and calibrations are usually performed based on a look-up table in the digital image processing computer 22 that executes a transform on the digital color image.
It is well known in the art that once such a digital image reproduction system has been set up to perform satisfactorily (for example in the factory), the performance of the components may drift or the properties of the print media may change, thereby necessitating the need for periodic calibration in the field. One representative prior art approach to calibrating a color image reproduction system in the field is disclosed in U.S. Pat. No. 5,107,332 issued Apr. 21, 1992 to Chan.
The method employed by Chan is illustrated schematically in FIG. 2. Initially, a transform F.sub.old is generated in the factory, which fully calibrates the color image reproduction system. This transform 26 is stored in the image processing computer 22. Later, in the field, when the system needs to be recalibrated, a sample test pattern 28 supplied by the manufacturer (called TP.sub.in) is scanned and processed through the system to produce an output test pattern 30 (called TP.sub.out). The digital image RGB produced by the scanner 20 when the test pattern is scanned is temporarily stored. The output test pattern 30 is then scanned by the scanner 20 to produce a second digital image RGB', which is also temporarily stored. The two stored digital images RGB and RGB' are then used to generate a matrix H (32) relating TP.sub.in to TP.sub.out. The matrix H is then employed to update (34) the transform F.sub.old to produce a new transform F.sub.new 36. Finally, the old transform (F.sub.old) is replaced in the computer 22 with the new transform (F.sub.new). This system color calibration is able to calibrate color reproduction systems in which the input images to the system and the output images from the system are printed on the same media (e.g. the same paper stock using the same dye or ink set as the original). In general, this method is not applicable to color reproduction systems in which the input and output images are printed on different media. This is because a scanner senses color differently from the way in which the human eye sees color. Two color patches printed on different media which appear the same to the human eye may be sensed differently by a scanner, and vice versa. For this reason, a color reproduction system which is calibrated based on an input target T.sub.in and an output target T.sub.out of the same media using the same dye sets may not improve color reproduction for input images on different media using different dye sets. For example, when the input to the color image reproduction system is a traditional photographic color print and the printer of the system is a thermal color printer using thermal paper and a thermal dye set, the prior art method of calibration will not be totally effective.